Solar and wind energy provide renewable, non-polluting energy sources, as opposed to conventional non-renewable, polluting energy sources, such as coal or oil. Because of this, solar and wind energy have become increasingly important as energy sources that may be converted into electricity. For solar energy, photovoltaic panels arranged in an array typically provide the means to convert solar energy into electrical energy. Similar arrays may be implemented for harvesting energy from wind or other natural energy sources.
In operating a photovoltaic array, maximum power point tracking (MPPT) is generally used to automatically determine a voltage or current at which the array should operate to generate a maximum power output for a particular temperature and solar irradiance. Although MPPT for the entire array is relatively easy to perform when the array is operating under ideal conditions (i.e., the same irradiance, temperature and electrical features for each panel in the array), when there are mismatches or partially shaded conditions, MPPT for the array as a whole is more complicated. In this situation, MPPT techniques may not provide accurate results due to relative optima of the multi-peak power-to-voltage characteristics of the mismatched array. As a result, only a few of the panels in the array may be operating ideally. This causes a drastic drop in power production because, for an array that includes strings of panels, the least efficient panel in a string determines the current and efficiency for the entire string.
Because of this, some photovoltaic systems provide a DC-DC converter for each panel in the array. Each of these DC-DC converters performs MPPT to find a maximum power point for its corresponding panel. However, currently proposed systems that implement such a distributed MPPT process generally do not propose the use of centralized MPPT control at the DC-AC conversion stage in conjunction with the distributed MPPT control. Instead, either distributed MPPT or centralized MPPT is implemented. As such, the interaction between the MPPT controller embedded in the DC-AC stage and the MPPT controllers in the DC-DC converters for each of the panels remains a design issue for any system implementing both types of MPPT control.
If the DC-DC converters lack specific features that allow the different types of MPPT control to work in conjunction with each other, then those DC-DC converters may only be suitable for operation without centralized MPPT control. In particular, dynamic interaction between distributed MPPT control and centralized MPPT control can lead the system to oscillations and can result in the panels operating away from their maximum power points. Also, if no synchronization is provided between the two different types of MPPT controllers, the DC-DC converters may start operating before the DC-AC stage starts operating. In this situation, the DC-AC stage is unable to dissipate or convert the power provided by the DC-DC converters, causing the string voltage to grow indefinitely and possibly cause damage to some components of the system. Furthermore, if no communication is provided between the MPPT controllers and the inverter, an islanding event or a grid disconnection would result in a condition in which the DC-DC converters continue to generate power while the DC-AC stage is no longer sinking power. This would result in uncontrolled growth of the DC-AC stage's input voltage.